In the field of ink jet printing, it is of prime importance that the ink droplets be ejected in rapid manner for high speed operation while maintaining the ink droplets in a condition which allows the droplets to dry upon contact with the record medium so as to avoid smearing of the ink, and at the same time keep the droplets in a wet condition so as to prevent drying of the droplets at the print head nozzle. The nozzle may become clogged by reason of inactivity or intermittent periods of operation or by use of a "dry" ink.
Certain inks having a slow-drying characteristic have been used in print heads along with the use of absorbent-type paper so that the paper can take care of part of the problem of the slow-drying ink. A disadvantage of the use of such slow-drying ink and absorbent paper is that the behavior of the ink and paper causes irregular dot patterns and distorted characters.
In the case of machine-readable characters such as certain bar codes used on high-quality papers as labels and vouchers, it has been common to use the quick-drying inks developed on the basis of organic solvents such as ketone or alcohol. However, such inks having an organic solvent base tend to dry, crust or thicken at the nozzle of the print head or within the print head itself, whereby the operation of the printing unit is impaired or otherwise affected.
Representative prior art in the area of conditioning the ink in an ink jet printer is shown in German Pat. No. 2,428,460, issued to IBM Corporation, corresponding, to U.S. Pat. No. 3,839,721 issued to W. H. Chen et al. on Oct. 1, 1974 which printer provides a tub that is filled with a vapor-generating liquid and is slidable in relation to the print head. During pauses in printing or non-printing, the nozzles are caused to be exposed to the vapor or to the liquid movement of the tub therepast so as to prevent the ink from drying in the nozzles.
U.S. Pat. No. 3,346,869 issued to J. J. Stone on Oct. 10, 1967 shows a nozzle cover or pad having a silicone coating with the pad being movable over the nozzle to prevent the ink from drying and also to serve as a lubricant for the tip of the nozzle.
U.S. Pat. No. 3,596,275 issued to R. G. Sweet on July 27, 1971 discloses a fluid droplet recorder having a reservoir of rinse water controlled by a valve to wash any residual ink from the nozzle when the recording operation is stopped.